The choice of encoding channels is constrained by more than perceptual precision, with one major constraint being the congruency of its metaphors [1]}. A channel must be consistent with (and convey) the concepts that they encode – for example, conveying quantity with area – serving as type of “affordance" for the data to show how it can be used (e.g., a push plate vs. a pull-bar for a door)  [2]}.
